This research proposal is based on the premise that the structure (crystallite size, morphology, internal strain, strength of bonding) and composition of the inorganic phase or dental enamel (principally an apatite) have a jointly fundamental (although not exclusive) roles in its caries susceptibility or caries resistance. The immediate objectives are: (1) to characterize the enamel from (a) different species of teeth (bovine, human, baboon, dog, etc); (b) from pre-eruptive, deciduous and mature human teeth of known ages; (c) teeth from subjects in high phosphate and in 'normal' diets, (d) teeth from areas of high cariogenic and high cariostatic trace elements; (2) to determine the comparative solubility of above enamel apatites in organic acids; (3) to establish the role of carbonate in the caries susceptibility of human enamel by determining the effect of its incorporation on the solubility as related to its effect on structural properties (size, morphology, strain); (4) to determine the effects of reportedly cariostatic and cariogenic trace elements on the structure and solubility of synthetic apatites; (5) to define the structural and compositional factors relating to the caries susceptibility of human teeth. X-ray diffraction, infrared absorption and atomic absorption analytical techniques will be used on the synthetic and dental apatites. BIBLIOGRAPHIC REFERENCES: LeGeros, R.Z. The unit-cell dimensions of human enamel apatite: effect of chloride incorporation. Arch. oral Biol. 20:63-71(1974). LeGeros, R.Z., Shirra, W.P., Miravite, M.A., LeGeros, J.P. Amorphous calcium phosphates: synthetic and biological. Coll. internationaux CNRS No. 230-Physico-Chimie et cristallographie des apatites d'interet biologique. pp. 106-115 (1975).